Thumbnails from server

This field cannot be defined in this scope (static).

The OwnCloudClient depends on an OC Account; the ThumbnailsCache is a single instance for all the existing OC Accounts. "Saving" just the last needed client will implies problems.

Right now, this is causing the bug #691; the first time the app is installed, while the account is being created, the InitTask is triggered with a NULL account, resulting in mClient = null; subsequent tasks to generate thumbnails will do nothing because of this NULL value.

The changes needed are:
- Move mAccount field from InitDiskCacheTask to ThumbnailGenerationTask; set it appropiately in its constructor.
- get an OwncloudClient instance in the doInBackground method of ThumbnailGenerationTask, not in InitCacheTask